PATH SWITCH WITH SERVICE CONTINUITY IN A LAYER-2 UE-TO-NETWORK RELAY
An indirect-to-indirect path switching procedure is proposed for transferring operation of a remote user equipment (UE) from a first relay UE to a second relay UE, while preserving continuity of the user's service(s) offered by a cellular network through the first and second relay UEs, in a layer 2 UE-to-network relaying architecture.
This application is filed under 35 U.S.C. § 111(a) and is based on and hereby claims priority under 35 U.S.C. § 120 and § 365 (c) from international application No. PCT/CN2021/125682, entitled “Indirect-to-indirect path switch with service continuity in a layer 2 UE-to-Network relay,” filed on Oct. 22, 2021. This application claims priority under 35 U.S.C. § 119 from Chinese application 202211257989.5 filed on Oct. 13, 2022. The disclosure of each of the foregoing documents is incorporated herein by reference.
TECHNICAL FIELDThe disclosed embodiments relate generally to wireless network communications, and, more particularly, to path switch in a layer 2 UE-to-Network relay in 5G new radio (NR) wireless communications systems.
BACKGROUNDIn a UE-to-network relaying environment, a so-called “remote UE” receives service from a cellular network via an intermediary “relay UE”, using a sidelink interface (for example, a PC5 interface in 3GPP systems) for communication between the remote UE and the relay UE. The protocol stacks between the two UEs and the network may be structured in various ways. If a “layer 2” relay architecture is considered, then the relaying relationship is mediated by an adaptation layer that functions as a sublayer of layer 2 of a protocol stack, e.g., located between the RLC and PDCP layers.
A UE may operate in direct communication with a network node such as a base station (gNodeB or gNB) while in coverage, referred to as a “direct path” service. Alternatively, a UE may function as a remote UE in a UE-to-network relaying relationship, obtaining service from the network via communicating directly with a relay UE, which then communicates directly with a gNB, referred to as an “indirect path” service. A remote UE may be in or out of network coverage. When the remote UE is in network coverage, it may experience poor link conditions that degrade its service quality on the direct path, and therefore it may prefer to operate on an indirect path through a relay UE. In a “single-hop” relaying environment, where only one relay UE is permitted to operate between a remote UE and the cellular network, the relay UE is in network coverage by definition. However, in a “multi-hop” relaying environment, where one relay UE may communicate with another relay UE rather than directly with the network, some relay UEs may also be out of network coverage. A setting where at least one of the involved UEs (for instance, a remote UE) is out of coverage, while at least one of the involved UEs (for instance, a relay UE) is in coverage, may be referred to as a partial-coverage scenario.
Due to various events such as physical mobility and/or changing radio conditions, a remote UE may initially be served by a first relay UE, and later find that a second relay UE can offer better service. In such a situation, it is advantageous for the remote UE to be able to perform a path switch operation, in which it relocates its service to use the second relay UE instead of the first relay UE. It is naturally preferable for the path switch operation to offer service continuity, i.e., to operate in such a manner that the user service is not interrupted as a result of the path switch.
In a layer 2 UE-to-network relaying environment, it is advantageous for the remote UE to be able to switch its service from operating through a first relay UE to operating through a second relay UE, a process which may be referred to as an indirect-to-indirect path switch. Furthermore, the path switch operation should allow continuity of user services.
SUMMARYAn indirect-to-indirect path switching procedure is proposed for transferring operation of a remote user equipment (UE) from a first relay UE to a second relay UE, while preserving continuity of the user's service(s) offered by a cellular network through the first and second relay UEs, in a layer 2 UE-to-network relaying architecture.
Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.
Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.
Such indirect-to-indirect path switch may be construed as a form of handover, in which the source and target are relay UEs, rather than cells/gNBs as in a conventional handover operation. Thus, for instance, the remote UE may be provided with a reconfiguration instruction, such as an RRCReconfiguration message of a radio resource control (RRC) protocol, that defines a configuration for the remote UE to operate with the target relay UE. The reconfiguration instruction may be sent by a serving network node and delivered to the remote UE via the source relay UE.
In the example of
In principle the gNB may trigger the path switch at any time, but under typical circumstances the gNB may be expected to trigger a path switch in response to receiving one or more measurements from the remote UE, with the measurements indicating that the target relay UE can be expected to offer better service than the source relay UE. For example, a measurement report may be triggered based on an event defined by “Candidate relay UE signal strength exceeds serving relay UE signal strength by a threshold”, an event defined by “Serving relay UE signal strength is below a threshold”, an event defined by “Candidate relay UE signal strength is above a threshold”, an event defined by “Serving relay UE signal strength is below a first threshold and candidate relay UE signal strength is above a second threshold”, and so on. The signal strength referred to in the event definitions may be a measure of signal strength on a sidelink interface, also known as a PC5 interface. The “serving” relay UE is synonymous with the source relay UE, and the “candidate” relay UE is synonymous with the target relay UE.
Similarly, for wireless device 211 (e.g., a remote UE), antennae 217 and 218 transmit and receive RF signals. RF transceiver module 216, coupled with the antennae, receives RF signals from the antennae, converts them to baseband signals and sends them to processor 213. The RF transceiver 216 also converts received baseband signals from the processor, converts them to RF signals, and sends out to antennae 217 and 218. Processor 213 processes the received baseband signals and invokes different functional modules and circuits to perform features in wireless device 211. Memory 212 stores program instructions and data 220 to control the operations of the wireless device 211.
The wireless devices 201 and 211 also include several functional modules and circuits that can be implemented and configured to perform embodiments of the present invention. In the example of
A layer 2 UE-to-network relaying architecture is well positioned to offer an “indirect-to-indirect” path switch procedure. In a layer 2 architecture, the upper layers of the protocol stack—for instance, an internet protocol (IP) layer, an SDAP layer, and/or a PDCP layer—terminate between the remote UE and nodes of the network. This means that the state of these upper layers (for example, an IP address of the remote UE) can be maintained even as the remote UE's data path switches between different relay UEs, making service continuity feasible without requiring special behaviour from the upper layers, such as handling mechanisms in the application layer to deal with changing IP addresses.
In step 2, the gNB makes the decision to trigger the path switch operation. This decision may be based on criteria specific to the gNB implementation. In step 3, the gNB brings the target relay UE to a connected state (for instance, an RRC_CONNECTED state of an RRC protocol). It is noted that the source relay UE may already be in a connected state, due to needing a connection with the gNB to transmit the relayed measurement report at step 1, and thus there is no need to bring the source relay UE to a connected state. However, the target relay UE may initially be in any protocol state, including, for example, an RRC_INACTIVE state of an RRC protocol or an RRC_IDLE state of an RRC protocol.
In step 4, the gNB sends a first reconfiguration instruction (for example, an RRCReconfiguration message of an RRC protocol) to the target relay UE, configuring the target relay UE to communicate on a sidelink interface with the remote UE. The first reconfiguration instruction may, for instance, contain configurations for one or more protocol layers for communication on the sidelink interface with the remote UE, such as a PHY layer, a MAC layer, an RLC layer, and/or a SALP or SLAP layer (i.e., PC5-ADAPT layer). The first reconfiguration instruction may also, for instance, contain configurations for one or more protocol layers for communication on a Uu interface between the target relay UE and the gNB, such as a PHY layer, a MAC layer, an RLC layer, and/or a SALP or SLAP layer (i.e., Uu-ADAPT layer).
In step 5, the gNB sends, via the source relay UE, a second reconfiguration instruction (for example, an RRCReconfiguration message of an RRC protocol) to the remote UE, configuring the remote UE to stop communication with the source relay UE and to start communication with the target relay UE. In some embodiments, step 5 may be realised by two separate reconfiguration instructions, such as a first RRCReconfiguration message instructing the remote UE to start communication with the target relay UE and a second RRCReconfiguration message instructing the remote UE to stop communication with the source relay UE. In other embodiments, step 5 may be realised by a single reconfiguration instruction.
In step 6, the remote UE and the target relay UE establish a link on the sidelink interface (for example, a PC5 link on a PC5 interface). It is noted that step 6 may not be necessary, if, for example, the remote UE and the target relay UE are already exchanging sidelink communication for other purposes besides relaying. However, even in such a case, the remote UE and the target relay UE may still opt to establish a separate link for the purpose of relaying. The details of step 6 may follow procedures in the existing art for the establishment of a PC5 link, including, for example, the exchange of a sequence of messages of a PC5 signalling (PC5-S) protocol to establish the link, the transmission of an RRCReconfigurationSidelink message of a PC5 radio resource control (PC5-RRC) protocol to configure the PC5 protocol layers between the remote UE and the target relay UE, and so on.
In step 7, the remote UE transmits, via the target relay UE, a handover complete indication (for example, an RRCReconfigurationComplete message of an RRC protocol) to the gNB. At this point, the “handover” portion of the path switch has completed, and the gNB is aware that the remote UE can be served through the target relay UE. In step 8, the gNB sends to the source relay a third reconfiguration instruction (for example, an RRCReconfiguration message of an RRC protocol), configuring the source relay UE to release its configuration for communication with the remote UE. The third reconfiguration instruction may, for instance, contain release indications for one or more protocol layers on the sidelink interface between the source relay UE and the remote UE. A release indication for a protocol layer may take the form of the absence of a configuration for the protocol layer, an explicit release instruction, and so on.
In step 9, the source relay UE and the remote UE exchange signalling to release their link on the sidelink interface (for example, a PC5 link on a PC5 interface). The details of step 9 may follow procedures in the existing art for the release of a PC5 link, including, for example, the exchange of a sequence of messages of a PC5-S protocol to release the link, the transmission of an RRCReconfigurationSidelink message of a PC5-RRC protocol to release the PC5 protocol layers between the remote UE and the source relay UE, and so on. In step 10, relayed data are transmitted between the remote UE and the gNB via the target relay UE.
It is noted that not all steps of
Alternatively, the “same cell” (or “same gNB”) constraint can be enforced by the network, i.e., the serving gNB may trigger a path switch operation to a particular target relay UE only if the target relay UE has the same serving gNB as the source relay UE. If the relay UE is in RRC_CONNECTED, it is obvious whether it has the same serving gNB, but in RRC_INACTIVE or RRC_IDLE, a candidate relay UE may perform cell reselection to a different cell at any time, and may thus be served by a different gNB, unknown to the remote UE's serving gNB. The necessary information on the relay UE's serving gNB can be made available to the remote UE's serving gNB with specific operating constraints on the system: the relay UE is always maintained in RRC_CONNECTED (and in this case step 3 of
With respect to the relationship between “same serving cell” and “same serving gNB” criteria, if the serving gNB maintains more than one cell, it can make the stored RRC contexts of the remote and/or relay UEs available to all the cells of the serving gNB. Thus, the intra-gNB flow of
In step 1 of
Based on identifying the target relay UE by a CN identifier, the AMF determines the connection management state of the target relay UE. If the target relay UE is in a connected connection management state (for example, a CM-CONNECTED state), steps 3-5 may be skipped. If the target relay UE is in an idle connection management state (for example, a CM-IDLE state), steps 3-5 are needed. In step 3, the AMF sends a paging instruction to the relay gNB, and in step 4, the relay gNB transmits a corresponding paging message over the air (for example, a Paging message of an RRC protocol). In step 5, the target relay UE responds to the page by sending a message to the relay gNB. The message may, for instance, comprise a request to establish an RRC connection. After step 5, the relay gNB may forward a message to the AMF indicating that the target relay UE is available, such as a non-access-stratum (NAS) message sent by the target relay UE in association with step 5 (not shown in the figure).
In step 6, the target relay UE may safely be assumed to be in a CM-CONNECTED state at the relay gNB. In step 7, the AMF sends to the remote gNB an indication of the identity of the relay gNB; this indication may be a new NGAP message. The indication may use non-UE-associated signalling. In step 8, the remote gNB begins a handover procedure for the remote UE by sending a handover preparation message to the relay gNB. The handover preparation message may be a message of an Xn application protocol (XnAP). The relay gNB evaluates the RRC protocol state of the target relay UE. If the target relay UE is in an RRC_CONNECTED state, steps 9 and 10 may be skipped. If the target relay UE is in an RRC_INACTIVE state, steps 9 and 10 are needed. In step 9, the relay gNB transmits a RAN paging message over the air (for example, a Paging message of an RRC protocol, in which the target relay UE is identified by an Inactive Radio Network Temporary Identifier (I-RNTI)). In step 10, the target relay UE responds to the RAN paging message, for instance, by requesting resumption of an RRC connection by the relay gNB. Step 10 may comprise multiple messages of an RRC protocol, such as an RRCResumeRequest message from the target relay UE to the relay gNB, an RRCResume message from the relay gNB to the target relay UE, and an RRCResumeComplete message from the target relay UE to the relay gNB (not shown in the figure). This operation may be in accordance with legacy methods of RRC connection resumption.
In step 11, the target relay UE may safely be assumed to be in an RRC_CONNECTED state at the relay gNB. In step 12, the relay gNB accepts the handover that was previously requested by the remote gNB, by sending a handover preparation acknowledgement to the remote gNB. The handover preparation acknowledgement may be an XnAP message. The handover preparation acknowledgement message may also be referred to as a handover accept message.
In step 13, steps 4-10 of
In step 6, the target relay UE may safely be assumed to be in RRC_CONNECTED state at the new gNB. In step 7, the anchor gNB rejects the handover that was requested by the remote gNB, providing, along with the rejection indication, an indication of the identity of the new gNB. The indication of the identity of the new gNB may be included in a rejection message. The rejection message may be an XnAP message. In step 8, the remote gNB sends a handover preparation message to request a handover of the remote UE to the new gNB. In step 9, the new gNB, which serves the target relay UE, accepts the handover, potentially providing configuration information that can be used to configure the remote UE for communication with the target relay UE.
Step 10 subsumes steps 4-10 of
As discussed above, it is preferred that the path switch procedure should offer service continuity to the user, i.e., an existing service should be able to continue uninterrupted.
In step 5, the target gNB sends to the target relay UE a first reconfiguration command (for example, a first RRCReconfiguration message) instructing the target relay UE to add the remote UE as a served remote. In step 6, the source gNB sends to the remote UE, via the source relay UE, a second reconfiguration command (for example, a second RRCReconfiguration message) instructing the remote UE to add the target relay UE as a serving relay and to remove the source relay UE as a serving relay. It is noted that steps 5 and 6 may take place in any order. In step 7a, the remote UE stops transmitting uplink (UL) data towards the source gNB via the source relay UE, and in step 7b, the source gNB stops transmitting downlink (DL) data towards the remote UE via the source relay UE. However, after steps 7a and 7b, UL data may continue to be generated and buffered at the remote UE, and DL data may continue to arrive from the CN at the source gNB. In step 8, the source gNB sends a sequence number (SN) status transfer message to the target gNB, conveying the status of the PDCP layer at the source gNB when over-the-air data transmission has been stopped; the SN status transfer message may be an XnAP message. The SN status may indicate one or more UL PDCP service data units (SDUs) that the remote UE will need to retransmit to the target gNB, via the target relay UE, when the path switch is completed. In step 9a, the source gNB forwards to the target gNB any DL data that arrives from the CN for the remote UE, and in step 9b, the target gNB buffers any such DL data. In step 10, the source relay UE delivers to the remote UE any remaining buffered DL data (for instance, data that arrived before the reconfiguration command in step 6, but that were not yet delivered successfully to the remote UE). In step 11, the remote UE and the target relay UE establish a PC5 link; this establishment may comprise a procedure for PC5-S link establishment and/or PC5-RRC connection establishment, which may subsume multiple messages whose details are not shown in the figure. It is noted that steps 8-11 may take place in a different order from the order shown in the figure; for instance, the source remote UE may deliver buffered data to the remote UE while the source gNB and the target gNB are carrying out steps 8 and 9a/9b. Steps 9a and 9b may take place on an ongoing basis for as long as DL data continue to arrive at the source gNB.
In step 12, the remote UE sends a handover complete message (for instance, an RRCReconfigurationComplete message) to the target gNB via the target relay UE, signifying that from the remote UE perspective the handover is complete. In step 13, the target gNB sends a handover success message to the source gNB; the handover success message may be an XnAP message. In step 14, the remote UE begins transmitting UL data, via the target relay UE, to the target gNB; this step includes delivering any UL data that were buffered at the remote UE after step 7a. In step 15, the target gNB transmits to the remote UE, via the target relay UE, any DL data that were buffered in step 9b. In step 16, the source gNB transmits to the source relay UE a third reconfiguration command (for instance, a third RRCReconfiguration message), indicating to the source relay UE to release the remote UE. In step 17, the remote UE and the source relay UE perform a PC5 link release procedure; this release may comprise a procedure for PC5-S link release and/or PC5-RRC connection release, which may subsume multiple messages whose details are not shown in the figure. It is noted that steps 16 and 17 may take place asynchronously with respect to steps 14 and 15. In step 18, the target gNB triggers a path switch operation with the CN, causing the user data path for the remote UE to be transferred from the source gNB to the target gNB; the path switch operation may take place in accordance with a legacy handover procedure. In step 19, the source gNB delivers to the target gNB an end marker, indicating that data forwarding from the source gNB to the target gNB has completed. In step 20, the target gNB sends to the source gNB a UE context release message; the UE context release message may be an XnAP message. In step 21, the target gNB begins transmitting DL data to the remote UE via the target relay UE, comprising any data that were received from the source gNB before step 19 as well as any new data arriving from the CN. It is noted that, as a result of the SN status transfer message in step 8, the target gNB is aware of what UL PDCP SDUs are missing (i.e., were not received by the source gNB) and can induce retransmission of the missing UL PDCP SDUs by legacy methods, such as sending a PDCP status report to the remote UE. Thus, the handover/path switch procedure can be lossless with respect to UL data sent by the remote UE. In the DL direction, all DL data delivered from the CN to the source gNB are either forwarded by the source relay UE (step 10) or forwarded to the target gNB (step 9a), buffered at the target gNB (step 9b), and sent to the remote UE (step 15), while all DL data delivered from the CN to the target gNB are sent to the remote UE (step 21). Thus, the handover/path switch procedure can be lossless with respect to DL data sent by the remote UE. Accordingly, the procedure may be seen to provide service continuity.
Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.
Claims
1. A method performed by a remote user equipment (UE) in a mobile communication network, comprising:
- communicating with a first relay UE on a first sidelink interface;
- sending a measurement report to a source gNB, wherein the measurement report comprises measurement results for at least one of the first relay UE and a second relay UE;
- receiving a first reconfiguration instruction via the first relay UE for communicating with the second relay UE on a second sidelink interface;
- receiving a second reconfiguration instruction for releasing the communication with the first relay UE; and
- communicating with the second relay UE on the second sidelink interface after sending a reconfiguration complete indication to the network via the second relay UE.
2. The method of claim 1, wherein the first reconfiguration and the second reconfiguration instructions are contained in a single reconfiguration message.
3. The method of claim 1, further comprising:
- establishing a second link with the second relay UE on the second sidelink interface based on the first reconfiguration instruction; and
- releasing a first link with the first relay UE on the first sidelink interface based on the second reconfiguration instruction.
4. The method of claim 1, wherein the measurement report is triggered based on an event that is conditioned on a measured quantity of signals from at least one of the first and the second relay UEs.
5. The method of claim 4, wherein the measured quantity is a measurement of signal strength or signal quality.
6. The method of claim 4, wherein the event conditions comprise the measured quantity of signals from the first relay UE being below a first threshold.
7. The method of claim 4, wherein the event conditions comprise the measured quantity of signals from the second relay UE being above a second threshold.
8. The method of claim 4, wherein the event conditions comprise the measured quantity of signals from the second relay UE exceeding the measured quantity of signals from the first relay UE by a threshold.
9. The method of claim 1, wherein the first relay UE and the second relay UE are served by the same source gNB.
10. The method of claim 1, wherein the remote UE receives the first reconfiguration message from the source gNB that serves the first relay UE, and wherein the remote UE sends the reconfiguration complete indication to a target gNB that serves the second relay UE.
11. A method performed by a source base station (gNB), comprising:
- determining to perform a path switch for a remote user equipment (UE) from a source relay UE to a target relay UE;
- identifying a target gNB that serves the target relay UE;
- preparing the target gNB for performing the path switch for the remote UE;
- sending a first reconfiguration instruction to the remote UE, wherein the first reconfiguration instruction comprises a configuration for communicating with the target relay UE on a target sidelink interface; and
- sending a second reconfiguration instruction to the remote UE, wherein the second reconfiguration instruction comprises an instruction to release a configuration for communicating with the source relay UE on a source sidelink interface.
12. The method of claim 11, wherein the first reconfiguration and the second reconfiguration instructions are contained in a single reconfiguration message.
13. The method of claim 11, wherein the source gNB and the target gNB are the same base station.
14. The method of claim 13, further comprising:
- sending, prior to the sending of the first reconfiguration instruction, a paging message to the target relay UE to transition to a connected protocol state.
15. The method of claim 13, further comprising:
- sending, prior to the sending of the first reconfiguration instruction, a third reconfiguration instruction to the target relay UE for communicating with the remote UE on the target sidelink interface.
16. The method of claim 11, further comprising:
- sending, subsequent to the sending of the second reconfiguration instruction, a fourth reconfiguration instruction to the source relay UE for releasing a configuration for communication with the remote UE on the source sidelink interface.
17. The method of claim 11, further comprising:
- sending, to an access and mobility function (AMF), a message requesting an identity of the target gNB;
- receiving, from the AMF, a message indicating the identity of the target gNB; and
- sending, to the target gNB, a handover preparation message that comprises a request to connect the remote UE with the target relay UE.
18. The method of claim 11, further comprising:
- sending, to an access and mobility function (AMF), a message requesting the identity of a third gNB;
- receiving, from the AMF, a message indicating the identity of the third gNB;
- sending, to the third gNB, a first handover preparation message comprising a request to connect the remote UE with the target relay UE;
- receiving, from the third gNB, a handover reject message containing an identity of the second gNB; and
- sending, to the second gNB, a second handover preparation message comprising a request to connect the remote UE with the target relay UE.
19. A remote user equipment (UE) in a mobile communication network, comprising:
- a sidelink interface that communicates with a first relay UE;
- a measurement circuit that provides a measurement report to a source gNB, wherein the measurement report comprises measurement results for at least one of the first relay UE and a second relay UE;
- a receiver that receives a first reconfiguration instruction via the first relay UE for communicating with the second relay UE on a second sidelink interface, wherein the receiver also receives a second reconfiguration instruction for releasing the communication with the first relay UE; and
- a transmitter that sends a reconfiguration complete indication to the network via the second relay UE, wherein the UE communicates with the second relay UE on the second sidelink interface.
20. The UE of claim 19, wherein the first reconfiguration and the second reconfiguration instructions are contained in a single reconfiguration message.
Type: Application
Filed: Oct 19, 2022
Publication Date: Apr 27, 2023
Inventors: Nathan Edward Tenny (San Jose, CA), Xuelong Wang (Beijing)
Application Number: 17/969,525